Tangent galvanometer
From Free net encyclopedia
A tangent galvanometer is an instrument used for the measurement of current. It works on the basis of tangent law of magnetism.
Construction
A TG consists of a circular coil of insulated copper wire wound on a circular non magnetic frame. The frame is mounted vertically on a horizontal base provided with levelling screws on the base. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. The compass box is circular in shape. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0° to 90°. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax a plane mirror is mounted below the compass needle.
Theory
When current is passed through the TG a magnetic field is created at its corners given by <math>B={\mu_0 nI\over 2r}</math> where I is the current in ampere, n is the number of turns of the coil and r is the radius of the coil.
If the TG is set such that the plane of the coil is along the magnetic meridian i.e. B is perpendicular to <math>B_H</math> (<math>B_H</math> is the horizontal component of the Earths magnetic field), the needle rests along the resultant. From tangent law, <math>B = B_H Tan\theta</math>, i.e.
- <math>{\mu_0 nI\over 2r} = B_H tan\theta</math>
or
- <math>I=({2rB_H \over {\mu_0 n}})Tan\theta</math>
or <math> I=K Tan\theta</math>, where K is called the Reduction Factor of the TG.